Passive vs active OT security testing: which to choose

Passive vs active OT security testing: when to choose each

Choose passive monitoring first. An operator or integrator who must know what is on the OT network before touching it should start with passive monitoring to build an asset inventory and identify likely vulnerabilities, because NIST SP 800-82 Rev. 3 notes that passive scanning adds no traffic, which suits sensitive OT devices. Passive tools can identify communicating devices along with manufacturer, part number and firmware version, which gives the team the facts it needs to plan any later active work. Active testing comes afterward, in planned outage windows, on replicas or in offline tooling validation, and most programs combine both approaches. This follows the point NIST SP 800-115 makes that organizations often combine techniques to reach an in-depth assessment at an acceptable level of risk.

The limits of passive monitoring matter as much as its strengths. Passive scanning cannot see silent devices, such as controllers that rarely communicate, and it cannot see inside encrypted traffic, so an inventory built only from passive data is incomplete. Confirm with the monitoring vendor which protocols it decodes and how it handles encrypted sessions in your environment. Also confirm how long a full passive pass will take on your network, since NIST SP 800-82 Rev. 3 says it often takes days. Plan manual reconciliation against asset lists, engineering drawings and physical walkdowns to close those gaps before concluding that the inventory is complete.

Before any active scan or test touches a controller, confirm three things. First, the team should have validated the scanning tool in an offline environment, as NIST SP 800-82 Rev. 3 advises. Second, device-specific checks should follow passive learning, so the active step is limited to what the passive data justified. Third, the work should be scheduled during a planned OT outage whenever possible, because NIST SP 800-82 Rev. 3 warns that active scans may cause device instability or interfere with process state, affecting safety and integrity. Ask any testing provider to show how it will handle these steps in writing before work starts.

How passive and active OT testing compare on goals, scope and risk

This table compares passive monitoring and active scanning of operational technology across goal, network impact, device risk, visibility, time, skills and output. Read each row as a planning question: the more a method touches the process, the more outage planning, offline validation and vendor input it needs.

Factor	Passive OT testing	Active OT testing
Primary goal	Build an inventory of communicating devices and observe traffic to spot exposures, without sending probes	Confirm specific vulnerabilities and configurations by querying devices, after passive discovery has shown what is present
What touches the network	Listens to existing traffic, such as mirrored switch ports or taps, and adds no traffic according to NIST SP 800-82 Rev. 3	Sends scan packets or device-specific requests, so the test itself becomes network traffic on control networks
Device impact	Minimal, because the method adds no traffic to sensitive OT devices	Can cause device instability or interfere with process state, potentially affecting safety and integrity per NIST SP 800-82 Rev. 3
Visibility limits	Cannot see silent devices or encrypted traffic, so gaps must be closed by other methods	Can reach devices that do not talk on the network, but results depend on what each device answers and may mislead if unsafe probes are skipped
Time to complete	Often takes days, since it depends on how long the monitored traffic takes to reveal each device	Depends on device count, scan design and outage windows, so confirm the schedule with the operator before work starts
Skills needed	OT network knowledge, protocol familiarity and the ability to read traffic and asset data without disrupting it	Deep knowledge of each device and its vendor-specific communication, plus change control and process safety awareness
Output produced	Asset inventory with manufacturers, part numbers and firmware versions where identifiable, plus passive findings to validate	Validated vulnerability and configuration findings, with evidence from device responses and a record of what was not tested
When to choose it	First step on any live OT network, and whenever devices cannot tolerate any added traffic	After passive discovery, in planned outages or on offline replicas, when a specific finding must be confirmed

How passive monitoring identifies OT assets without adding traffic

Passive monitoring identifies OT assets by listening to traffic that already crosses the industrial network, which is why NIST SP 800-82 Rev. 3 notes that it adds no traffic and suits sensitive devices. A sensor attached to a mirrored switch port (a SPAN or mirror port) reads communications between a PLC, an HMI, an engineering workstation and a historian, then records addresses, protocol behavior and device details. For communicating devices, that metadata can reveal manufacturer, part number and firmware version, which gives the asset inventory and later vulnerability matching a starting point. The engineering team should confirm which switches, VLANs and conduits the sensor will see before any hardware is installed, because a sensor that sees only the corporate side of a boundary will produce an incomplete inventory that looks complete.

The blind spots matter as much as the gains. Passive scanning cannot see silent devices, meaning equipment that only responds when polled or that sits idle during the observation window, so a field device that communicates once a month may never appear. Encrypted traffic limits how much identity and firmware detail the sensor can extract, so the inventory may hold only the device address and a guessed type. NIST SP 800-82 Rev. 3 also notes that full passive coverage often takes days, so a short capture taken during a quiet shift will understate the asset count. Ask the provider to state the observation period, the protocols the sensor decodes, and how silent assets will be tracked for follow-up in a planned outage window.

Passive does not mean risk-free. A span port misconfigured to send production traffic back onto a control network, a sensor with a network interface that can transmit, or a tap installed on a link without the owner's approval can each disrupt operations or expose the network to new paths. Confirm with OT engineering who approves the port change, which interfaces on the sensor are enabled for management, how the sensor is patched, and who can log in to it. Test the sensor and its configuration in an offline environment first, as NIST SP 800-82 Rev. 3 advises for scanning tools, and record the change in the control system's change process before connecting anything to production.

How safe active scanning builds on passive findings

The sequence NIST SP 800-82 Rev. 3 implies starts with quiet discovery, so that every active step is aimed at devices the team already understands and has tested. Each step produces a record that decides whether the next step can safely run.

Start with passive network monitoring
Deploy passive monitoring on a span or tap of the OT network and let it observe traffic for as long as it takes to see the communicating devices. The OT security lead or the monitoring provider does this work, and the team confirms it is done when the inventory lists devices with manufacturers, part numbers and firmware versions where the traffic reveals them. NIST SP 800-82 Rev. 3 notes that passive scanning adds no traffic and that it often takes days to complete, so plan for that duration. The output is a baseline inventory with known blind spots, including silent devices and encrypted sessions that passive monitoring cannot see.
Reconcile the inventory with operations
Compare the passive inventory with the asset register, network diagrams and the operations team's list of controllers, historians and engineering workstations. Operators and engineers confirm which devices are in service, which are spares, and which are safety-relevant. The check is complete when every device is classed as in service, retired, or unknown, and each unknown device has an owner. Anything unknown stays out of active scope until someone who understands its process role has reviewed it.
Classify devices by safety and process impact
Rank each device by how a disruption would affect the process, safety systems and availability. Controllers that run safety functions or time-critical loops sit at the top, and devices with known fragility, such as older controllers with limited vendor support, are flagged for the most restrictive handling. The OT engineer and the plant safety lead sign off the list. The output is a scope register that marks each device as passive-only, eligible for device-specific active checks, or excluded from active work.
Validate scanning tools in an offline environment
Before any tool touches production, run it against a replica or lab set of representative devices with the same firmware where possible. NIST SP 800-82 Rev. 3 advises testing vulnerability scanning tools offline first. The tester records which probes cause timeouts, resets, memory growth or changes in device state, and the team writes down the settings that were safe. The check is complete when the test log shows no unexplained device behavior and the approved settings are recorded in the change ticket.
Use device-specific communication for deeper detail
For devices in the eligible category, switch from generic probes to the protocols and commands the device vendor documents, so the tool can gather firmware, configuration and vulnerability details without sending unexpected traffic. NIST SP 800-82 Rev. 3 describes this as safe active scanning, built on what passive learning already showed. The engineer performing the work keeps a step-by-step log, and the check is complete when each queried device returns the expected data and the operations team confirms no alarms or process changes occurred.
Schedule active work in planned outages
Place any active scan that could affect device state into a planned OT outage whenever possible, with operations, safety and maintenance staff present and a rollback plan agreed. NIST SP 800-82 Rev. 3 says active scans may cause instability or interfere with process state, which can affect safety and integrity. The change owner confirms that the outage window, the fallback procedure and the decision authority are written down before the work starts. Ask the testing provider how they will stop mid-run if a device behaves unexpectedly.
Keep the gas utility lesson in scope
Enforce network boundaries so that a test aimed at the corporate network cannot route into SCADA segments. NIST SP 800-82 Rev. 3 recounts a penetration test of a gas utility's corporate network that reached a part of the network connected to SCADA, locked up the system and stopped gas being sent through pipelines for four hours. The security team confirms the rules of engagement exclude OT segments unless explicitly approved, and that firewall rules or network segmentation block unplanned paths. The check is complete when the test plan shows the segment boundaries and the operations team has signed off on them.
Feed results back into monitoring and remediation
Combine the active findings with the passive inventory, update the asset register and open remediation tickets for confirmed vulnerabilities, with owners and compensating controls where patching is not possible. Set passive monitoring to alert on the new behavior the tests introduced, so the team can confirm the activity was expected. The check is complete when the register reflects the current state of each device, every finding has a disposition, and the next passive baseline is scheduled.

What passive and active testing find, and what each misses

Passive monitoring and active testing surface different things in an industrial network, and neither covers everything. The cards below pair each finding with the method that surfaces it and the gap that method leaves, so teams can decide where each method is safe to run and what needs a planned outage window or a replica.

Asset inventory

Passive monitoring builds an inventory from traffic already on the wire, so it adds no load to sensitive controllers. It can identify communicating devices with manufacturer, part number and firmware version where the traffic reveals them. It misses silent devices that never talk on the monitored segment, such as standby controllers or equipment on a span port that was not mirrored. Check the switch mirror coverage, then confirm the inventory against the asset register and physical walkdowns.

Communication paths

Passive capture shows which devices talk to which, on which protocols and at what intervals, which is the basis for finding unexpected links between IT and OT zones. It cannot see encrypted traffic in content, and it often needs days of capture before the picture is complete. Active discovery can confirm a path quickly, but only after a safe-scan plan is agreed. Ask the provider which paths were observed passively and which were confirmed actively.

Known vulnerabilities by firmware version

Matching identified firmware versions against vulnerability records is a passive, analysis-only step once the inventory is known. It does not prove that a vulnerability is reachable or exploitable on that device. Version strings can also be wrong or unchanged after patching, so the match needs vendor advisories. Confirm with the asset owner and the vendor whether a listed version is in fact the running build.

Configuration weaknesses

Weak settings such as default credentials, open services, unnecessary remote access and unsigned logic are usually found by reviewing device configuration, not by sending traffic at it. Passive monitoring can hint at some of these, such as clear-text logins, but it rarely confirms them. Configuration checks need an engineer with access to the device or its backups, and they should run inside a planned window if any login or read touches the controller.

Exploitable paths

Demonstrating that an attacker can move from the corporate network into a control zone is the work of active penetration testing, which sends real traffic and exploits. This is where the risk concentrates. NIST SP 800-82 Rev. 3 recounts a corporate-network test that reached a SCADA-connected segment, locked the system and halted gas flow for four hours. Require a written scope that bars entry into control networks unless a replica or outage window has been approved.

Process-state impact

Only active testing can show whether a scan or exploit changes the process, through device instability or interference with control logic. Passive methods add no traffic and cannot reveal this risk, so their results say nothing about safety margins. Validate tools offline first, as NIST SP 800-82 Rev. 3 advises, and run any test that could affect the process during a planned outage. Ask the provider which tests they would stop if a controller reset or alarm appeared.

Which approach fits which OT environment

When a plant has no current inventory, the first job is to learn what is there without disturbing it. NIST SP 800-82 Rev. 3 describes passive scanning as adding no traffic, which suits sensitive OT devices, and it can identify communicating devices along with manufacturers, part numbers and firmware versions. Its limits matter here: it cannot see silent devices or encrypted traffic, and it often takes days to complete, so the inventory is only as complete as the observation window.

Lead with passive monitoring: Mirror or tap traffic at existing switches and let the monitor build the asset list over a window long enough to capture polling cycles and maintenance activity.
Follow with safe active scanning: Use the passive results to target device-specific queries for silent or unidentified devices, following the safe active approach NIST describes.
Cross-check with site records: Compare observed devices against engineering drawings, purchase records and change logs to find gaps that traffic never revealed.
Record what passive missed: Keep a list of silent devices and encrypted segments so the gap is visible to the operator rather than assumed clear.
Confirm before starting: Ask the operator which switches can be mirrored, whether any SPAN port load is acceptable, and who owns each network segment.

Before any active step, confirm with the operator which devices are safety-relevant and which vendor procedures apply to them.

Using replicas and planned outage windows for active tests

Active validation belongs on a replica or test bench wherever one is available, and inside a planned outage window when it is not. NIST SP 800-82 Rev. 3 warns that active scans may cause device instability or interfere with process state, which can affect safety and integrity, and advises scheduling them during planned OT outages whenever possible. A replica lets the team run the device-specific checks, exploit validation and tool trials that could destabilize a live controller, and see the effect on a copy of the system before anything touches production.

A replica only helps if it matches the production system closely enough that a result on it means something. Ask the asset owner and the control system vendor to confirm that the copy has the same controller firmware versions, the same logic and configuration, the same network layout and the same communication paths to historians, engineering workstations and remote access. A bench that runs an older firmware build or skips a field network can pass a test that the live device would fail, or the reverse. Also confirm in writing whether the vendor supports testing on a copy, because some vendor terms, warranties or support agreements limit what may be run against their equipment, and a test that breaks a vendor-supported configuration can leave the asset owner without recourse.

When no replica exists, plan the active work as a controlled event rather than an open-ended scan. Agree the window with operations, name the people who can halt the test, define the stop conditions in advance, and confirm that the passive baseline from earlier in the engagement is current so the team knows what should be talking on the network during the outage. Ask the provider how the test tools were checked in an offline environment before use, since NIST SP 800-82 Rev. 3 advises that step. The duration and cost of building a replica or booking an outage depend on the site, the vendor and the scope, so request those figures from the provider and the plant as part of a written quote.

What you receive from each approach

This table compares the deliverables a passive monitoring engagement and an active OT assessment typically produce, row by row. Read it to see which outputs you can rely on before scope is agreed, and which ones need planned outage windows or replica testing to produce safely.

Deliverable	Passive monitoring	Active testing with safety controls
Asset inventory with manufacturers and firmware	Identifies communicating devices with manufacturers, part numbers and firmware versions where traffic reveals them; silent devices stay missing	Adds device-specific queries after passive learning to confirm details, but only where the device's behavior is known to be safe
Communication map	Built from observed flows over time; the NIST SP 800-82 Rev. 3 note that passive scans often take days means the map is only as complete as the observation window	Can confirm open services and paths on targeted devices, with the risk that a query disturbs a controller
Vulnerability list	Candidate findings matched to observed firmware and versions; not proof that a device is exposed	Findings validated against the live or replica device, with the test method recorded for each
Validated exploitability and safety notes	Not produced; passive work does not attempt exploitation	Each validated item records the method, the expected process impact and the stop conditions used during testing
Test plan and rules of engagement	Monitoring scope, sensor placement, data handling and who may access captured traffic	Written plan covering target list, timing in planned outage windows, abort criteria and operator on-call; NIST SP 800-115 Appendix B template is a useful starting point
Remediation guidance	Prioritized list of network and asset issues for the owner and the vendor to review	Fix steps, compensating controls and retest criteria, with notes on which fixes need a vendor-approved change
Offline tool validation record	Evidence that the capture and parsing tools were checked in an offline environment before production use	Evidence that scanning tools were tested offline first, as NIST SP 800-82 Rev. 3 advises

What drives cost and effort for each approach

Passive monitoring and active testing of industrial networks cost different things, and the drivers differ. Passive work scales with sensors, network size and time on the wire, while active work scales with fragile devices, replica availability and the people who must stand by during a test. Ask every provider to price these drivers separately.

Sensor count and placement

Passive monitoring depends on how many sensors are deployed and where they sit, such as span ports or taps on OT segments and at boundaries between IT and OT. Each sensor adds hardware or virtual capacity, installation work on live networks and ongoing tuning. Confirm in writing who installs the sensors, who owns the hardware after the engagement, and whether placement requires network changes that the asset owner must approve. The placement decision drives most of the passive cost.

Monitoring duration and recurrence

NIST SP 800-82 Rev. 3 notes that passive scanning often takes days to complete, so the monitoring window is a direct cost driver. Longer windows raise analyst time and infrastructure hours, and a single snapshot misses devices that communicate only during batch cycles or maintenance. Ask whether the quote covers one window or recurring monitoring, and who reviews alerts between reports.

Network size and analysis effort

Larger networks produce more flows, more asset records and more findings to triage. Analysis effort depends on how well passive data is matched to manufacturer, part number and firmware records, and how many devices cannot be identified because they are silent or use encrypted traffic. Ask the provider how it validates asset records, how unidentified devices are handled, and whether the asset inventory is delivered in a format your team can maintain.

Device count and device sensitivity

Active testing cost rises with the number of in-scope controllers, PLCs, RTUs and HMIs, and with how sensitive each one is. Devices that can be safely probed with device-specific methods take less time than those that must be tested manually or excluded. Request a device-by-device scope list with a sensitivity classification, and ask which devices the provider will not touch and why.

Replicas and offline tool validation

Testing against a replica of production controllers is often the largest one-time cost in active OT work, because it requires matching hardware, firmware and logic. NIST SP 800-82 Rev. 3 advises testing vulnerability scanning tools in an offline environment before using them in production OT, which adds validation effort and lab time before any live work starts. Ask whether the quote includes building or borrowing a replica, and who pays for the offline tool validation.

Outage windows and operator staffing

Active scans should be scheduled during planned OT outages whenever possible, so outage windows set both the calendar and the staffing. Plant operators, control engineers and safety staff must be present, and an outage that runs over can cost more than the test itself. Ask who schedules and staffs the window, whether the client pays for standby operators, and what the rollback and stop criteria are in the written scope.

How compliance frameworks and combined programs treat OT testing

The practical question for an OT owner is not which testing method wins but how passive monitoring and active testing fit into one program that an assessor will accept. NIST SP 800-115 states that organizations often combine techniques to reach an in-depth assessment at an acceptable level of risk, and it suggests regularly scheduled scanning interspersed with periodic penetration testing. For OT, that pattern usually means continuous passive monitoring for inventory and anomaly visibility, with active work limited to planned windows, replicas or safe active methods that NIST SP 800-82 Rev. 3 describes.

CIS Controls v8.1 Safeguard 18.4 asks that security measures be validated after each test so that detections cover the techniques the testers used. In an OT setting, this means the passive sensors and alerting rules should be checked against what a planned active test actually did on the network, and the results recorded as evidence. Treat this as a sequencing step: run the test in the agreed window, then confirm that the monitoring saw the relevant traffic and that any gaps are assigned owners and dates.

Which OT-specific requirements apply to your organization depends on your sector, your regulator and the frameworks your assessor uses, and this page does not set those for you. Confirm with your assessor or regulator which controls they expect, how they want passive and active evidence presented, and whether they accept replicas as a substitute for production testing. For sequencing an assessment against ISA/IEC 62443 with an attack-style test, read the IEC 62443 assessment vs OT penetration testing comparison and the OT penetration testing vs IT penetration testing comparison before agreeing a schedule.

Questions buyers ask before choosing OT testing

Is passive monitoring alone enough to secure an industrial network?

No, passive monitoring alone is rarely enough, but it is the right first step. NIST SP 800-82 Rev. 3 says passive scanning adds no traffic, which suits sensitive OT devices, and it can identify communicating devices along with manufacturers, part numbers and firmware versions. It cannot see silent devices or encrypted traffic, so it gives an incomplete inventory. Teams usually combine passive discovery with periodic, carefully planned active checks, and with physical and process reviews that no network tool performs on its own. Confirm with your provider which vulnerability checks rely on passive data and which need device contact.

How long does passive OT coverage take to build?

Plan for days, not hours. NIST SP 800-82 Rev. 3 notes that passive scanning often takes days to complete, because it can only observe the traffic that the process produces during the monitoring window. Quiet devices, such as those that communicate only during maintenance or on a particular cycle, may not appear until that activity happens. Ask the provider how long the capture window will run, which operating periods it covers, and how it will record devices that were not seen, so that gaps are documented rather than assumed away.

Can active testing run safely on live controllers?

Only with strict controls, and often not at all during production. NIST SP 800-82 Rev. 3 warns that active scans may cause device instability or interfere with process state, which can affect safety and integrity, and says they should be scheduled during planned OT outages whenever possible. It also describes safe active scanning, which first learns about equipment passively and then uses device-specific communication to gather more detail. Before any active step, ask the provider to show how each action was chosen for the specific device, and whether it was validated on a replica or offline test bench.

What must our team provide before OT testing starts?

The engagement depends on your internal records, so prepare them early. Typical inputs include:

An asset list or inventory, including vendors, models and firmware where known
Network diagrams showing zones, conduits and any links to corporate IT
Planned outage windows and the process owner who can approve them
Named operator sign-off for each active step, plus a contact who can stop testing
Rules of engagement that define prohibited actions and escalation paths

NIST SP 800-115 includes a rules of engagement template in Appendix B that can serve as a starting point. Confirm the exact list with your provider in writing.

What drives the cost of OT testing?

The main drivers are the number of zones and sites, the number and variety of controllers, how much active work is in scope, and how many outage windows the work needs. Replicas or test benches add cost but reduce the risk of touching production. Passive monitoring costs depend on sensor placement, the capture period and the analysis effort. Active testing costs rise with the care needed per device, the coordination with operators, and any reruns after an outage is missed. DeployOpen does not publish a price list. A quote follows from written scope, so ask for each phase to be priced separately.

Can our own team run passive OT monitoring in-house?

Often yes, if the team has OT network knowledge and the authority to work with operations staff. Passive tools need a clear placement plan on mirrored or tapped traffic, an agreed change process, and someone who can interpret industrial protocols and asset data. The recurring work is continuing analysis, tuning and reviewing new devices, not only the first deployment. Teams without that skill set often start with a supported assessment and then decide which parts to keep. DeployOpen plans, implements and documents self-hosted platforms inside the client's own infrastructure, so ownership of the data and tooling stays with you.

What should we ask for in an OT testing quote?

Ask for a written scope that names each zone, asset class and test type, and states which steps are passive and which touch devices. Request the outage windows assumed, the staff you will need from operations, and how safety stops are handled. Ask how replicas or offline tool validation will be used, what the deliverables are, and how findings are reported and retested. Confirm how the provider handles change control during the engagement, and what happens if an outage is cancelled. Compare quotes on that scope, not on headline price alone.

How DeployOpen scopes, delivers and supports OT security testing

DeployOpen is a Chainscore Labs company that plans, implements, tests and documents self-hosted platforms inside the client's own infrastructure, including security testing. For passive versus active OT work, we agree the monitoring and testing boundaries in writing before any traffic reaches a controller, so the choice between listening and probing is made deliberately and recorded.

How we scope

Written scope names which OT segments are monitored passively and which, if any, may receive active checks.
Scope records the planned outage windows, named process owners and stop conditions agreed with operations staff.
Rules of engagement set out permitted methods, source hosts, timing and escalation contacts before testing begins.

How we build

Passive monitoring sensors are deployed in the client's own network so that inventory builds without adding traffic.
Any vulnerability scanning tool is first tested in an offline environment before it is used near production OT.
Active steps follow a safe active approach: learn devices passively first, then use device-specific checks only where scoped.

How we hand over

Deliverables include an asset inventory, a list of observed communications and findings tied to the scoped segments.
Each finding notes whether it came from passive observation or an active check, so operations can judge the evidence.
Handover documents recommended monitoring baselines and the detections that should cover any techniques the testers used.

How we support

Ongoing support, upgrades and continuous testing are available as retainers after the initial engagement ends.
Retainers can cover periodic passive re-inventory and scheduled active checks during agreed outage windows.
Clients should confirm retainer terms, response expectations and change control in the written agreement before work starts.

Passive vs active OT security testing